In Fischer-Tropsch synthesis, synthesis gas (a mixture of CO and H2) is converted to a range of hydrocarbons (from normally gaseous to waxy material) and water. If a so-called shifting catalyst is employed, some of the product water reacts with CO according to the water gas shift reaction to form CO2 and H2. Iron-based catalysts typically have a high activity for the water gas shift reaction and are therefore regarded as shifting catalysts. However, some catalysts, for example those based on cobalt or ruthenium, do not have a high activity for the water gas shift reaction and therefore produce very small amounts of CO2 via the water gas shift reaction. These catalysts, known as non-shifting catalysts, typically have a CO2 selectivity of less than about 2%, i.e. less than about 2% of the total CO consumed in the synthesis process is converted to CO2.
The Fischer-Tropsch reaction per se consumes H2 and CO in a ratio of about 2, i.e. has a H2/CO consumption ratio or usage ratio of about 2. Depending on the selectivity towards light hydrocarbons, especially methane, this consumption ratio is pushed up to slightly above 2, e.g. 2.05. On the other hand, the conversion of some CO to CO2 via the water gas shift reaction tends to lower the usage ratio by consuming CO and producing H2. Therefore, the overall usage ratio of the synthesis gas is typically in the range of 2-2.1 for a non-shifting Fischer-Tropsch process, while it can be substantially below 2 for a shifting Fischer-Tropsch process. A drift over time in the product spectrum produced with a Fischer-Tropsch catalyst, e.g. an increase over time of the selectivity towards methane and lighter hydrocarbon products, can slightly affect the usage ratio. Furthermore, a change in the relative rates of the Fischer-Tropsch and water gas shift reactions over the catalyst can substantially change the usage ratio. Nevertheless, in commercial operations steps are normally taken to limit such variations, such as periodic regeneration or rejuvenation of the catalyst or online catalyst replacement yielding normally a substantially constant usage ratio for a commercial Fischer-Tropsch process.
It is well known that the selectivity behaviour of Fischer-Tropsch catalysts is strongly influenced by the H2/CO ratio, with lower ratios favouring the production of desired heavy products. For this reason, Fischer-Tropsch synthesis processes are sometimes operated at H2/CO ratios that are lower than the H2/CO usage ratio. It follows by simple mass balance, that the H2/CO ratio in the reactor outlet will then be different from the feed H2/CO ratio. It will be appreciated that when recycle around a Fischer-Tropsch reactor is then employed, the H2/CO ratio of the feed to the reactor becomes a difficult to control parameter as a result of positive feedback in the system. It would therefore be an advantage if a method could be found that facilitates operation with a stable H2/CO ratio.
For commercial applications, a Fischer-Tropsch reactor and its synthesis loop normally form part of a larger plant which, amongst others, includes a synthesis gas generation stage, such as gas reforming or coal gasification. The tail gas from the Fischer-Tropsch synthesis stage is often employed elsewhere in the larger plant. For example, some or all of the tail gas can be recycled to the synthesis gas generation stage in order to assist in producing a synthesis gas of the required composition (H2/CO ratio) by providing a CO2-rich feedstream. All or some of the tail gas can also be used for further chemical conversion downstream of the Fischer-Tropsch reactor. Due to the highly integrated nature of such petrochemical plants, and the fact that gaseous streams (with consequently limited capacity for buffering) are flowing between plant units or stages, stable operation of such integrated complexes present challenges. A method is therefore also required efficiently to achieve or facilitate this.
U.S. Pat. No. 7,776,932 discloses the control of an integrated Fischer-Tropsch process by determining the H2/CO ratio in both the feed to the Fischer-Tropsch synthesis stage (A1), as well as in the effluent from the Fischer-Tropsch synthesis stage (A2), and adjusting the H2 and/or CO in the synthesis gas feed to the Fischer-Tropsch synthesis stage to keep the difference between A1 and A2 essentially constant. This is achieved by controlling the operation of the synthesis gas generation stage, which adjusts the H2/CO ratio in the feed to the Fischer-Tropsch synthesis. In this process the ratios A1 and A2 are therefore allowed to vary, with the difference between A1 and A2 being maintained constant.
WO 2002/038699 teaches a method of controlling the feed H2/CO ratio to the Fischer-Tropsch synthesis reactor by recycling an H2 or CO2 containing stream to a reformer and feeding the reformed gas to the Fischer-Tropsch synthesis reactor.
US 2004/014825 discloses a system where two synthesis gas streams, one with an H2/CO ratio above 2 and one with an H2/CO ratio below 2, together form the feed to a Fischer-Tropsch reactor. The composition of the tail gas stream from the Fischer-Tropsch reactor is measured and the flow rate of one of the two feed streams is adjusted depending upon the composition of the tail gas stream. In one embodiment of the invention, the ratio of partial pressures of H2 and CO in the tail gas is used to adjust the flow rate of the two reactor feed streams. In other words, US 2004/014825 also teaches the control of the outlet H2/CO ratio by adjusting the overall inlet H2/CO ratio.
U.S. Pat. No. 5,023,276 teaches a method of controlling the H2/CO ratio of synthesis gas produced in an autothermal reformer, the method including the removal of CO2 from the reformer effluent, and recycling some or all of the CO2 back to the reformer inlet. Additionally, the effluent from a Fischer-Tropsch synthesis reactor can also be recycled to the reformer inlet. By controlling the proportions of the various feed streams to the reformer, the desired H2/CO ratio is obtained for the Fischer-Tropsch synthesis.
The above references therefore teach various methods of obtaining a synthesis gas with the desired H2/CO ratio from a synthesis gas generation stage in order to ensure an appropriate feed composition to a Fischer-Tropsch synthesis stage. These references further teach methods of control of a Fischer-Tropsch synthesis process which involve varying the inlet H2/CO ratio to the Fischer-Tropsch reactor in order to obtain a desired outlet H2/CO ratio.
Water formed in the Fischer-Tropsch synthesis process can have a detrimental effect on the catalyst and is one of the leading causes of catalyst deactivation. Usually there is a limiting value of water partial pressure above which serious catalyst deactivation occurs. This point normally represents an operating constraint for the process. This constraint can assume a variety of mathematical forms, and can be as simple as the absolute water partial pressure or the ratio of the water partial pressure to that of one or both of the reagents CO and H2. More complex functional relationships between water partial pressure and one or both of the reagents CO and H2, or a combination of constraints, can also be applied to ensure a safe operating window for the Fischer-Tropsch catalyst in terms of the water partial pressure.
It is not readily possible directly to measure the partial pressures of water and reagents inside or at the outlet of a commercial Fischer-Tropsch reactor. A broad range of components is typically contained in the Fischer-Tropsch reactor outlet, and includes unconverted CO and H2, as well as CO2, H2O and light hydrocarbons. This severely complicates quantitative analysis of such samples, as some components (water and hydrocarbons) are condensable at ambient conditions. It is also not easy to identify a detector that can quantitatively analyse such a variety of components. Clearly a method is required which allows for an accurate and fast determination of these partial pressures inside the Fischer-Tropsch reactor, as well as a quick response to make corrective actions as required.
Once certain operating parameters of the Fischer-Tropsch synthesis (e.g. total pressure, water content in the feed (i.e. conditions of water knock-out)) are essentially fixed at constant values, it is then possible to fairly relate the water partial pressure to the extent of conversion achieved inside the reactor (the so-called per pass conversion or single pass conversion). In addition, provided the Fischer-Tropsch synthesis has a reasonably constant usage ratio and feed gas H2/CO ratio, it is also possible to fairly relate a mathematical relationship between the partial pressures of water and the reactants to the per pass conversion. In other words, ensuring a safe operating window for the Fischer-Tropsch catalyst in terms of the water partial pressure is essentially reduced to controlling the per pass conversion. This critical per pass conversion can be determined by those skilled in the art from specified process conditions and constraints.
However, determining the per pass conversion achieved on an operating commercial Fischer-Tropsch process is not a straightforward matter as will be illustrated below. It will be appreciated by those skilled in the art that the conversion achieved in a Fischer-Tropsch reactor can be expressed in a variety of ways, e.g. in terms of the CO conversion, the H2 conversion or the (CO+H2) conversion. For purposes of illustration, the following explanation will be based on the CO conversion.
The per pass CO conversion of a Fischer-Tropsch synthesis process can be expressed as follows:
      χ    CO    =                              F          In                ⁢                  C          In          CO                    -                        F          Out                ⁢                  C          Out          CO                                    F        In            ⁢              C        In        CO            
where FIn and FOut are the total inlet and outlet volumetric flow rates of gas (at normal conditions) of the Fischer-Tropsch reactor, respectively, and CInCO and COutCO are the volumetric concentrations of CO in the inlet and outlet gas streams, respectively. Each of these four parameters needs to be measured independently and accurately in order to calculate the conversion, whereafter corrective action needs to be taken to ensure maximum conversion while not exceeding the upper conversion limit constraint. However, this is not trivial to achieve, especially not for large scale commercial plants where gas flow rates are very high. For example, flow meters need to be calibrated and also require some physical data from the gas stream (e.g. density), which can change with process variations. The CO concentration in the gas stream will usually be obtained by analysing one or more samples of the streams, e.g. on a gas chromatograph. A broad range of components is typically contained in such streams, especially in the Fischer-Tropsch reactor outlet, and includes unconverted CO and H2, as well as CO2, H2O and light hydrocarbons. This severely complicates quantitative analysis of such samples, as some components (water and hydrocarbons) are condensable at ambient conditions. It is also not easy to identify a detector that can quantitatively analyse such a variety of components. A further source of uncertainty in the calculated conversion is the principle of error propagation, which dictates that the errors contained in the individually measured quantities are amplified via the mathematical calculations performed. It is therefore clear that this approach of estimating the conversion achieved on a large scale reactor will be subject to significant error and uncertainty. A substantially larger safety margin then needs to be built in to avoid accidentally exceeding the operating limits, which implies that the process will generally not be operated close to the optimum operating point. It will therefore be an advantage if a method can be found that enables fairly estimating the per pass conversion.
It will be appreciated by those skilled in the art that the same problems are encountered when the object is to calculate and control the overall conversion achieved in a Fischer-Tropsch reactor, e.g. the overall conversion when recycle around the reactor is also taken into account.
In the design phase of a Fischer-Tropsch synthesis process, a broad range of conditions (e.g. pressures, H2/CO feed ratios, etc.) are normally explored in order to locate the optimum operating point for the design. During the design phase, the constraints with respect to water are applied in order to ensure that the catalyst operating limits are not exceeded, i.e. to ensure a safe operating window for the Fischer-Tropsch catalyst in terms of the water partial pressure. This typically also considers per pass conversion and overall conversion among other considerations. Commercial plants are however designed and operate to ensure operation at optimum performance. This optimum operating point or region will generally be close to the constraints related to the water partial pressure to maximise productivity and process efficiency. The challenge is therefore to operate a commercial plant as close to these limits as possible to achieve maximum process efficiency, while not exceeding applicable limits in order to protect the integrity of the catalyst. It will be appreciated that such a control philosophy is not trivial, amongst others due to the large scale at which such Fischer-Tropsch processes are operated. For example, the production capacity of a single Fischer-Tropsch slurry bubble column reactor is being pushed beyond 20 000 bbl/day. This means that the transient behaviour of the system, compounded by the enormous heat generation and complicated chemistry, is complex and difficult to predict. Large deviations from set points can result in runaway situations with serious detrimental effects.